Aircraft landing gear, amongst other support devices, can experience strains that might lead to catastrophic failure. During landing, a landing gear strut can be deformed due to strain in different directions. Subjective determination of when landing gear should be inspected or replaced may be over- or under-reported, leading to unnecessary inspections or a missed opportunity to inspect.
Strain is defined as the amount of deformation per unit length of an object when a load is applied. Strain is calculated by dividing the total deformation of the original length by the original length (L):Strain(e)=(ΔL)/L 
For a polysilicon piezoresistive type of element, the resistance is changed with load applied. When a strain is introduced, the strain sensitivity, which is also called the gauge factor (GF), is given by:
  GF  =                              Δ          ⁢                                          ⁢          R                R                              Δ          ⁢                                          ⁢          L                L              =                            Δ          ⁢                                          ⁢          R                R            Strain      
The most popular strain gauges are metal foil elements on polyimide film. Piezoresistive strain gauges have been developed that offer 100× improvement in gauge factor over metal foil elements. These are generally fashioned in the form of a Wheatstone bridge. The ideal strain gauge would change resistance only due to the deformations of the surface to which the gauge is attached. However, in real applications, temperature, material properties, the adhesive that bonds the gauge to the surface, and the stability of the metal all affect the detected resistance. Furthermore, the sensing range of usual strain gauges is limited by maximum stress allowed by the sensing element. For example, the maximum strain limitation of both types of strain gauge and for silicon strain gauges is 3,000 micro-strain. FIG. 1 shows fatigue limits on foil gauges. Even at 3,000 micro-strain they will start to shift at less than 10,000 cycles.